It is known that, in reverse osmosis, the solutions of salts or other solutes, notably those of low molecular weight, for instance sea water, maple water and others, are put into contact with a selective membrane and are subjected to a pressure. Contrary to what happens in the case of a conventional osmosis where there is an equilibrium of the solution on each side of the membrane, the reverse osmosis works in a way such that the solution having a low concentration and even a very very low concentration emerges on the side of the membrane opposite the original solution. In all, in order to reverse the normal osmotic flow from the side of the membrane where the solution is less concentrated towards the side where the solution is more concentrated, there is exerted on the solution to be treated a pressure differential which is higher than the osmotic pressure differential of the solutions in contact with the surfaces of the membrane. Now, it has been discovered that, during a reverse osmosis operation, the concentration at the interface membrane-solution to be treated was higher than the mean concentration on the high pressure side of the membrane. This abnormally high concentration at the interface becomes an obstacle to the quality of the product obtained because on one hand an important fraction of the salt or other particles in the solution, in contact with the membrane, is rejected and on the other hand, by recirculating, there is a reduction in the concentration at the surface of the membrane of the components of low solubility which can be tolerated without precipitating on the membrane.
Commonly, clogging is used to define all phenomena, other than temperature variation and compaction, which cause a lowering of a membrane's permeability to pure water. These phenomena are linked to the presence of solutes or other particles in suspension, notably colloids, bacteria, etc., which can settle at the surface or in the pores of the membrane. The clogging can be more or less rapid depending on the nature of the particles which are present and on their concentration at the surface of the membrane. To obviate the problems resulting from the clogging of the membrane, which is more or less reversible, we resort to a rinsing with cold pure water or with hot water, or to a cleaning.
In the production of maple syrup, maple water has always been evaporated until the syrup is obtained. Now, with the staggering increase in the price of energy, it has shown to be useful to effect such an evaporation from a solution which is more concentrated than the water obtained directly from the maple tree. To do so, we have resorted to reverse osmosis which rejects water which is almost pure and produces at the end a water which is more concentrated. As in other reverse osmosis cases, there is an important clogging problem of the membrane. Indeed, the maple water solutes are essentially sugars and minerals. Maple water contains also bacteria having a number which can vary from a few tens to many millions per ml. The solutes are almost totally retained by the membranes of the reverse osmosis or nanofiltration types, notably very close to a 100% for the sugars and more than 95% for the minerals. A fortiori, the particles in suspension, including the bacteria, are also retained. The sugar molecules, which are bigger, diffuse less rapidly than the ions, the most present type of minerals in solution. All of this favors, relatively, a greater accumulation of sucrose than of minerals at the surface of the membrane thereby resulting in an important clogging of the latter.
Nowadays, we have noticed that the best way to overcome at least partly the problem of the clogging of the membrane, was to have recourse to a recirculation of the liquid under reverse osmosis treatment. To do so, the liquid can be recirculated in the same pump or the recirculation can be achieved by way of an additional pump. In the system without recirculation, we have obtained a recovery of 19% (permeate flow:feed flow), and consequently, a large waste of water. If, on the other hand, we recirculate in the same pump, we obtain a mean recovery of approximately 24% but there is also a waste of water and energy. If we wish to have a good yield, we must then have recourse to three similar systems, one after the other, which is extremely costly. To obtain a good yield of approximately 75%, in a single operation, two pumps can be used with one of the pumps being used only for the recirculation. This alternative is however very costly in view of the presence of the two pumps.
It would be thus of interest to be able to use a system having a single pump, but which would have a recovery substantially improved with respect to what is presently known, and even comparable to the use of two pumps.
As to the technological prior art, the following documents must be mentioned even though they have nothing to do with the present invention:
U.S. Pat. No. 3,472,765 PA0 U.S. Pat. No. 3,505,215 PA0 U.S. Pat. No. 4,705,625 PA0 U.S. Pat. No. 4,773,991